Over the past several years, X-ray baggage inspection systems have evolved from simple x-ray imaging systems that were completely dependent on interpretation by an operator to more sophisticated automatic systems that can automatically recognize certain types of contraband. The more sophisticated inspection systems have employed single energy or dual energy X-ray radiation transmitted through or scattered from the examined baggage. Some systems have used a single view source detector arrangement, others have utilized a dual view or multi-view arrangements. The single or dual view systems usually scan baggage, as it moves on a conveyor, using a fan beam or a scanning pencil beam of X-rays in a fixed geometry. The multiview, Computed Tomography (CT) type systems usually scan stationary baggage in a fixed geometry of scan angles and process data corresponding to absorption of X-rays to reconstruct selected slices of the baggage.
At airports, the baggage inspection procedure is divided into at least three levels of inspection. A `level 1` system processes baggage rapidly, ideally at a rate of up to 1500 bags per hour. This system is located at a first inspection station and inspects all baggage; as such, it represents the first line of defense. The system rapidly scans baggage and automatically makes a decision based on its particular modes of detection and methodology. This methodology may be object based, e.g., mass density, effective atomic number (Z.sub.eff), or Compton X-ray scatter, or it may be bag-based, such as Nuclear Quadrupole Resonance (NQR), ion mass spectroscopy, vapor chemi-luminescence, or other techniques. Such systems are able, to some degree, to rapidly eliminate "clean" and non-suspicious baggage from the stream of passenger baggage and have been proven effective in detecting real threats. The number of bags that cannot be cleared at level 1 can range from 10% to 50% of the total number of bags. The clearing efficiency depends on the particular detection methodology and threat thresholds used in the system.
The rejected (i.e., non-cleared) bags are automatically sent to a `level 2` area. In the `level 2` area, an operator usually visually inspects an x-ray image of the rejected bag and attempts to determine whether a suspicious object inside the bag can be cleared based on its obvious shape. The operator searches the image for characteristic objects such as weapons, timing and detonation devices, wires, or other characteristics associated with contraband. The operator at this station can clear most, but not all of the rejected bags. The remaining baggage, which is usually 0.1% to 0.5% of the initial stream, is then sent on to a `level 3` inspection station which is usually a slower inspection device that uses a different technology.
Vapor or trace detectors (also called `sniffers`) and CT scanners have been used as `level 3` devices. A vapor or trace detector does not employ a penetrating type of radiation, but looks for traces of characteristic materials. It was suggested that, by careful placement inside a bag, a relatively large amount of explosives can pass undetected by the trace device. On the other hand, a CT scanner, which of course employs penetrating X-ray radiation, is usually successful in identifying explosives inside a bag especially when present in a large amount. The CT scanner basically measures a single parameter, that is, the mass density of the examined object. The CT scanner can be set up to communicate with the `level 1` device in order to interrogate a specific object identified within the baggage by that device. If a `level 1` X-ray scanner identifies threats on the basis of effective atomic number (Z.sub.eff) the additional density information from the CT scanner can significantly reduce the false alarm rate, but may not be able to eliminate it completely. However, CT scanners can be very expensive considering the low utilization at a `level 3` station.
On average, `level 3` devices clear less than half of the inspected objects, yielding 0.05% to 0.25% of the baggage to be sent to `level 4`. `Level 4` is defined as reconciliation with the owner. The reconciliation is often difficult, if not impossible. When the reconciliation is not possible the bag is confiscated, which causes a complaint from an unhappy passenger.
While the above system can perform successfully, there is still a need for a relatively low cost X-ray inspection device that can reliably detect various explosives (or other contraband) having different shapes and being located anywhere in the examined baggage. Such a device should have a high confidence and a low false alarm rate.